Design of a simple tool for quality assurance of stereotactic gamma knife radiosurgery procedures

Sajeev Thomas1

1

Keywords: radiosurgery, gamma knife, dose delivery, quality assurance, physics

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Abstract

     Measurements of quality and assurance testing are crucial.
     Our purpose was to design and develop a simple tool and direct method for addressing physicists concerns from imaging till treatment delivery for Leksell stereotactic Radiosurgery procedures.
     
     A Linear Target phantom has been designed using acrylic slabs of varying thickness connected using nylon rods and acrylic bolts using CNC machine. The phantom has drilled holes of varying length from 2 mm to 14 mm with 1 mm diameter separated at equal distances .Volume chambers are designed for simulating Target volumes ( Teflon for CT imaging and gels for MR Imaging) . Plates were designed for placing of 0.125 cc and pin point ion chambers. Plates of 3 cm x 3cm were designed for inserting Films for Dosimetry and provisions are made for placing TLD dosimeters for dose measurements. A special plate has been fixed with lead balls of 1mm diagonally for assessing the geometrical accuracy of stereotactic Angiographic imaging.The assembly was attached to the Leksell frame using a base plate and was tightened using platinum screws. A special plate has been fixed with lead balls of 1mm diagonally for assessing the geometrical accuracy of stereotactic Angiographic imaging.The assembly was attached to the Leksell frame using a base plate and was tightened using platinum screws. CT,MRI,DSA images of the phantom has been acquired using Philips Brilliance 16 CT scanner , Siemens Magnetom vision 1.5 Tesla MR unit and Siemens Neurostar DSA unit with appropriate indicator boxes using routine imaging protocols used in gamma knife Radiosurgery procedures.The images were exported to the Leksell Gammaplan through hospital network . Images were defined and registered in Gammaplan and stereotactic coordinates are evaluated.Skull frame measurements are taken and treatment plans are generated for Known doses .The phantom with Ion chamber and films are irradiated using APS system in Leksell 4c and measurements are repeated for a period of time.Using IAEA –TRS 398 protocol dose measurements were performed and FWHM were compared with known stereotactic values in Leksell Gamma Plan.
     Mean value of absolute maximum error estimated for MR images from Siemens Magnetom vision MR unit were 0.41 mm(X-Axis),1.53 mm(Y-Axis) and 2.02 mm(Z-Axis).Mean of absolute maximum error estimated using CT images from Philips Brilliance 16 CT scanner were 0.34mm (X-axis), 0.43mm (Y-Axis) and 0.75mm (Z-Axis).The mean maximum error in estimating the vector distances from the centre of the Leksell system(100,100,100) to known Targets were 0.22 mm and 0.80 mm for CT and MR images respectively. The percentages of maximum error in volume calculations were evaluated separately for CT and MR Image data. Isocentric accuracy, positioning accuracy of Trunnion and Automatic Positioning System (APS),absolute dose delivered at different treatment positions in Leksell Gamma Knife 4c were calculated and analyzed.
     We did not compare this method to other systems.
     This phantom can be used for evaluation and quality assurance of complete radiosurgical procedures  including multimodality imaging and treatment delivery. This technique is simple direct and may be adopted even in simple clinical set-ups where there is limited man power and minimum equipments. 
     This can be used for commissioning as well as routine quality assurance of the equipment and treatment plan in any Radiosurgery programs.


Acknowledgements

Project Roles:

S. Thomas ()